nm9 has it correct. Many guys think they are sucking the pan dry but they are not. The pickup is too far from the bottom of the pan and it runs out of oil.
This is for OE style pickups.
I see in the pictures, that makes a lot more sense. I didn't know the factory pickup had that little spacer on it.Not with the stock pickup. It has a 'shield' (for lack of a better word) on the bottom, spaced under the screen maybe 3/16", and an opening to the side where the oil actually enters. (Some aftermarket pickups do indeed pickup up through the bottom and they need the spacing.)
Adding more spacing under the spacing already provided by the stock pickup design just increases the chances of sucking in some air, which is YR's point.